Important characteristics for a bearing of a spindle motor used for magnetic recording apparatus such as a hard disk, or a bar code scanner, etc. are wear and abrasion resistance, vibration resistance, stability of rotation, and the like. Particularly, when rotating members and fixed members of the bearing contact with each other due to vibration, rotation of the spindle motor becomes unstable, which in turn seriously influences reliability and durability of the bearing. In order to avoid this, a technology for enhancing the thrust rigidity of a bearing is in demand.
FIG. 6 shows a hydrodynamic bearing disclosed in Japanese Patent Kokai (A) Publication No. 69715/1999, which could be one of solutions for the above problem. In FIG. 6, a shaft 12 is fixed to a housing 11. A cylindrical member 13 with a closed end on one side is fitted on an outer circumferential surface of the shaft 12 such that the cylindrical member 13 freely rotates. These two members form a bearing. A so called herringbone-shaped groove is formed on the outer circumferential surface of the shaft 12.
A rotor magnet 15 is attached to an outer periphery of the cylindrical member 13, and a stator 14 which forms an electromagnet is attached to the housing 11 so as to face the rotor magnet 15. As a coil wound around the stator 14 is energized, attraction/repellence force is developed between the stator 14 and the rotor magnet 15, which in turn drives the spindle motor. With the cylindrical member 13 rotating due to the driving force, relative motion between the outer circumferential surface of the shaft 12 and a facing inner circumferential surface of the cylindrical member 13 (i.e., radial bearing portion) is developed, which generates a radial hydrodynamic pressure. Air is introduced by an effect of the herringbone-shaped groove formed on the outer circumferential surface of the shaft 12, and is guided to the area between the closed end of the cylindrical member 13 and an upper surface of the shaft 12 (i.e., thrust bearing portion), and applies a pressure. As a result, the cylindrical member 13 is lifted up relative to the shaft 12.
A ring-shaped thrust member 16 is disposed to an outer circumferential portion of the cylindrical member 13, while a thrust retaining member 17 is disposed to the housing 11 at a position opposed to the thrust member 16. As the cylindrical member 13 is lifted up, the thrust member 16 moves closer to the thrust retaining member 17, and a thrust hydrodynamic pressure is generated between the two. This thrust hydrodynamic pressure balances with the pressure generated at the thrust bearing portion, whereby the bearing rotates stably in a non-contact condition.
However, in the case of the hydrodynamic bearing disclosed in Japanese Patent kokai (A) Publication No. 69715/1999, it is difficult to accurately adjust the squareness of the thrust member 16 and a gap with the thrust retaining member 17 after the cylindrical member 13 is inserted during assembly process. Further, since the thrust member 16 and the thrust retaining member 17 are additionally disposed, the structure becomes more complex, larger and heavier.
The purpose of the present invention is, therefore, to provide a bearing which solves the problem as described above, and which can be assembled into a simple structure with a high thrust rigidity and can obtain stable rotation.